Density Functional Theory Study On Oxidation And Reagent Adsorption Of Pyrite And Marcasite

The Fe-S minerals is an important chemical material raw material for the production of sulfur,which occupies an important position in agriculture,industry and national economic development.In addition,as one of the most common gangue minerals in sulfide ores,flotation separation of the Fe-S minerals from other minerals is often encountered.Pyrite and marcasite are the most representative minerals in the Fe-S minerals.Although they have the same chemical composition,they show different physicochemical properties due to different crystal structures,resulting in differences in their oxidation and flotation behavior.In this paper,the microscopic oxidation mechanism and flotation reagent adsorption mechanism of pyrite and marcasite were investigated based on density functional theory（DFT）and coordination chemistry.The main findings are as follows:（1）The Fe―S bond lengths in pyrite crystals are equal,while there are two types of Fe―S bonds with different bond lengths in marcasite crystals,one of which has very strong covalency,but the S―S bonds are relatively weak with very low covalency.The surfaces of pyrite（100）and marcasite（010）are the most stable cleavage planes,in which Fe atoms on pyrite surface are 5-coordinated and S atoms are 3-coordinated,while the surface Fe atoms of marcasite are 4-coordinated and the surface S atoms are 2-coordinated.The relaxation degree of Fe and S atoms on the outmost layer of marcasite is larger than that of pyrite.The density of states（DOS）calculation indicates that the activity of Fe atoms on marcasite surface is greater than that on pyrite surface.In addition,Fe atoms on pyrite surface have no spin,while the spin value of Fe atoms on marcasite surface is 0.12（?）,indicating that theπelectron structure of these two minerals are different.According to the coordination chemistry theory,the 3d orbitals of Fe atoms on pyrite surface have threeπelectron pairs,while theπelectron pair of Fe atoms on marcasite is less than three.（2）H₂O and O₂ molecular adsorption studies showed that the adsorption of H₂O on marcasite surface is stronger than that on pyrite surface,indicating that the marcasite surface is more hydrophilic,and the dissociative adsorption of O₂on marcasite surface is also stronger than that of pyrite.The results of H₂O-O₂ co-adsorption show that O₂exists on pyrite surface and dissociates at Fe sites,H₂O dissociates and adsorbs at S sites,while O₂ dissociates at S sites,H₂O does not dissociate.For marcasite,O₂ dissociates and adsorbs at both Fe and S sites,and H₂O adsorbs at Fe sites in molecular form.Under the multi-water-oxygen system,the oxidation of pyrite surface forms iron oxides and free SO_x groups,while the oxidation products of marcasite surface are Fe SO₄ and SO_x^2-groups.It is noteworthy that the S―S bond on marcasite surface is easily broken during oxidation.（3）The adsorption of xanthate,dixanthogen and dithiophosphate on the surface of pyrite and marcasite was studied.It is found that the hollow Fe―Fe sites on the surface are the optimal site for collector adsorption,and the adsorption of the same reagent molecule on marcasite surface is stronger than that on pyrite surface.Xanthates with different carbon chain lengths have little effect on the adsorption of pyrite,while the adsorption of marcasite increases with the enhanced carbon chain lengths of xanthates.The dissociative adsorption of dixanthogen occurs on marcasite,and the adsorption energy is merely-16.42k J/mol for molecular adsorption on pyrite surface.The adsorption of dithiophosphate on pyrite surface is much weaker than the adsorption on marcasite surface.When the collector is adsorbed on pyrite surface,the valence shell orbitals d²²_x-y and d_xy of Fe atoms are the main participating orbitals;while the main participating orbitals of marcasite is the d_z² orbital.And the adsorption of three kinds of collectors on the two surfaces is in the order of xanthate>dithiophosphate>dixanthogen.（4）The adsorption of lime,sodium hydroxide,sodium sulfide and cyanide on the surfaces of pyrite and marcasite shows that the O,S and C atoms of depressants interact with the surface Fe atoms to form bonds,so that the depressants are adsorbed on the mineral surface.The depression effect of the same depressant on marcasite is stronger than that on pyrite,on the contrary,the depression effect of cyanide on pyrite is stronger,and Ca²⁺in lime has a certain influence on the depression effect.（5）The difference of cyanide interaction:According to the theory of flotation coordination chemistry,the emptyπorbital in CN^-can acceptπelectron pairs of metal ions,while the Fe d orbital on pyrite surface possesses threeπelectron pairs,and the Fe d orbital on marcasite surface has fewer than threeπelectron pairs,and hence,CN^-has stronger interaction with pyrite than marcasite.